Crankshaft assembly

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Hi all,
I am fabricating a crankshaft to the plans below. I have made the main piece, have nearly finished the rear piece and the crank pin hopefully won’t be far behind. My query is about assembling the crankshaft and making sure that I get it alined properly. I assume that I will need to make some sort of jig to align them, does anybody have any ideas/suggestions?
Thanks
John Lukes
Swansea
UK

Member

I used to assemble three piece two cycle engine crankshafts and I made an assembly jig out of a die shoe set. That did an excellent job of aligning the two crankshaft halves but I still had to 'bump' the assembly to achieve acceptable alignment.

Well-Known Member

Does it really need a press fit? Lots of small engines with that style of crankshaft just use a snug fit that is self aligning. Two stroke engines with a pressed in crank pin need a jig like the pictures below. As was noted, they still need some straightening.

Member

That’s really interesting, I wondered if it would be self aligning, moreover, whether they would move out of alinement if they weren’t a tight fit. the crankpin and the holes in either side of the crankshaft are all specced to be the same size, so it will be a push fit, but not a fridge and oven fit!

Well-Known Member

A lot depends on the particular situation. I would need to see the whole design for good advice. If all the rear section of the crankshaft does is supply an auxiliary drive function and extra counterbalance, it doesn't need to be a press fit. For example, it drives a disk valve. This assumes two widely separated bearings on the front section of the crankshaft, unlike your design. You then have an overhung crank pin. The design with a pressed in pin and a bearing on the front and back crankshaft section is more compact. It isn't necessarily more stiff. We had to add an additional front bearing on a 35 cc engine to limit crankshaft flexing. Double bearings on the front and back crankshaft sections are common on our race engines.

The jig I made like the one pictured has very close fits (.001" or .025 mm clearance) on the sliding posts. I machined the top and bottom halves at the same time in the same setup for accuracy. It will hold the crankshaft sections close to final alignment, but some tweaking is still needed to get the sections to run within .001" (.025 mm) TIR. If you need to make your crankshaft a press fit, I would redesign it to use a constant diameter crank pin that presses into both sections. That way you have an easier to make crankshaft that can use a hardened crank pin. That allows a needle bearing connecting rod end that will be very durable. It will make drilling the oil holes more difficult. They wouldn't necessarily be needed with a needle big end bearing. Below is a picture of a twin cylinder crankshaft I built with a single crank pin using a jig.

Member

I have attached a schematic of the whole engine, it might give more perspective.
I don’t think that I would want to be redesigning or remaking the crank at this stage, it took a lot of effort and a few write offs on the way to get to the stage I’m at at the moment!
It seems to me that a jig will be the way to go to get this crank aligned accurately,could even be good fun designing and making it having said that I am very interested to hear any further thoughts you may have

Well-Known Member

The rear section of your crankshaft fills my definition of an auxiliary drive. It has two bearings for alignment and definitely doesn't need help from the crank pin fit. You can allow up to .003" (.075 mm) clearance on the pin in my opinion. That will make manufacture and assembly a lot easier. The front section is well supported by the sleeve and ball bearing. It doesn't need help from the rear ball bearing.

Member

Hi - nothing really to do with crankshaft construction but was interested to see on the whole engine drawing there is a pressure lubrication connection. How is this effected? The drawing does not show a collection in the crankcase sump.

Member

The rear section of your crankshaft fills my definition of an auxiliary drive. It has two bearings for alignment and definitely doesn't need help from the crank pin fit. You can allow up to .003" (.075 mm) clearance on the pin in my opinion. That will make manufacture and assembly a lot easier. The front section is well supported by the sleeve and ball bearing. It doesn't need help from the rear ball bearing.

Lohring Miller

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I’ll aim to do better than .003” but good to know. Somehow not having a good tight fit doesn’t seem right!
Thanks for your advise.

Member

Hi - nothing really to do with crankshaft construction but was interested to see on the whole engine drawing there is a pressure lubrication connection. How is this effected? The drawing does not show a collection in the crankcase sump.

Chris

PS What engine is it, looks interesting!

Click to expand...

Hi Chris,

It is a Chenery aero v twin from Hemmingway kits. My first engine build and I’m enjoying the challenge and learning lots on the way

Member

Why did you not just make the crankshaft in one piece as is normal with engines of this size. All you need is a Keats angle plate on your lathe faceplate. Much simpler and easier to make correctly than cobbling one up from pressed together parts.
Ken C

Well-Known Member

Engines are built with pressed together crankshafts to benefit the rod and its big end bearing. That tends to be the limiting element in performance. Needle bearings and light rods still fail after relatively short periods of high rpm running. An article on one solution is at http://namba.com/content/library/propwash/2012/April/#/11

Member

The OP is making a one ended crankshaft for a relatively low powered engine. This would normally be machined in one piece. The only reason to use a pressed-in crank pin would be for ease of manufacture in an engine such as this. Except that getting the correct press fit requires precise machining and measurement, which negates the ease of manufacture. Of the 50 or so engines I have built, of 40 different designs, all bar one has a single crankshaft bearing, and all of the other cranks are machined in one piece, the easiest way bar far.